Don't open up chosen on option keyup.

When embedding chosen in a chrome extension, the keyboard shortcut
includes the option key. If a chosen instance happens to be focused at the
time, releasing the option key opens it, but shouldn't.

coffee/lib/abstract-chosen.coffee

@@ -245,7 +245,7 @@ class AbstractChosen
       when 27
         this.results_hide() if @results_showing
         return true
-      when 9, 38, 40, 16, 91, 17
+      when 9, 38, 40, 16, 91, 17, 18
         # don't do anything on these keys
       else this.results_search()